okay well do everything I said above and You can play with higher graphics settings cause of your card but just tweak stuff around see what works best, idk how hard it is to overclock that cpu but look it up and try to.
Also un-park your cpu cores, you can look up how.
Set arma to high priority in task manager
get CCleaner and clean your pc also check for viruses with a anti virus program even tho they barley work.
Try using parameters it might help you.
Check your CPU temps while in game or out of game that was a problem for me for a very long time.
High recommend you optimize your windows for gaming just look it up on youtube.
This guide below is very helpful